Polyvinyl chloride can be processed conventionally with plasticizers to form pasty, moldable compositions, so-called pastes or plastisols. Polyvinyl chloride suitable for such plastisol preparation can be conventionally obtained by polymerization in aqueous emulsion in the presence of inorganic catalysts or by polymerization in aqueous microsuspension with preliminary homogenization in the presence of oil-soluble catalysts.
It is furthermore known that the thermostability of the polyvinyl chloride powder product, as well as the rheological properties of plastisols produced therefrom and the foam characteristics of foam materials manufactured mechanically or chemically from the plastisols, can be improved by specific additives (German Pat. No. 1,119,513; DAS (German Published Application) 2,245,958; German Pat. No. 2,126,950).
Initially, such additives were added to the finished plastisol. (See U.S. Pat. Nos. 2,966,470 and 2,861,963). Later, the procedure was altered and these additives were added to the polyvinyl chloride dispersion prior to spray-drying. As a result, after the spray-drying step, granules having a homogeneous distribution of the additives were obtained. (See German Pat. No. 2,126,950, claim 5; and German Pat. No. 1,119,513).
However, when the additive is immiscible with the polyvinyl chloride dispersion, inhomogeneous mixtures are obtained due to immediate phase separation during the admixture of the additive. After the spray-drying step, powders result which likewise exhibit an inhomogeneous distribution of the additive. Due to this nonuniform distribution of the additives in a portion of the powder (Comparative Experiments 1 and 2 herein), the addition of such materials to the powders is ineffective for the intended purposes. Furthermore, if the additive compound causes the sprayable dispersion to coagulate to even a slight extent, clogging of the nozzles is frequently observed (Comparative Experiment 1 herein). This phenomenon requires interruptions of the process in order to effect expensive cleaning operations.
When the additive is not only immiscible with the polyvinyl chloride dispersion but also has a coagulating effect on the dispersed polyvinyl chloride particles, the respective material can only be incorporated into the powder with considerable consumption of time by adding the material to the dispersion and subsequently spray-drying (Comparative Experiment 2 herein). Constant clogging of the nozzles requires considerable time for cleaning operations. Furthermore, due to the frequent interruptions, the temperatures in the spray dryer cannot be maintained at the precisely desired value, whereby the grain properties and rheological characteristics of the products are altered.